Wheel offset and backspacing explained
Offset and backspacing both answer one question: where does the wheel sit relative to the part of the vehicle it bolts to? Get either one wrong and the wheel rubs on the suspension or sticks out past the fender.
Start at the hub face
The wheel bolts onto a flat surface on the hub, called the mounting surface or hub face. Everything about wheel position is measured from there. Inboard means toward the vehicle (toward the springs, brakes and steering parts). Outboard means toward the outside (toward the fender).
Offset, also called ET
Offset is the distance in millimetres from the wheel’s centerline to its mounting face. It is stamped on the back of most wheels as ET followed by a number, such as ET35.
- Positive offset (ET+35): the mounting face is toward the outside, so the rest of the wheel sits further in toward the suspension. Most factory wheels on modern cars are positive.
- Zero offset (ET0): the mounting face is exactly on the centerline.
- Negative offset (ET−12): the mounting face is toward the inside, so the wheel sits further out. This is the “deep dish” look, common on lifted trucks.
The simplest way to remember it: a lower offset number moves the wheel outward, and a higher one tucks it inward.
Backspacing
Backspacing is the distance from the mounting face to the back edge of the wheel. It is popular in the truck and off-road world, usually in inches. It carries the same information as offset once you know the wheel width:
backspacing (in) = (wheel width + 1) ÷ 2 + offset (mm) ÷ 25.4 offset (mm) = (backspacing − (wheel width + 1) ÷ 2) × 25.4
The extra inch in the formula allows for the rim flanges, because the stamped width is measured between the flanges and the wheel is wider overall. Here are some worked examples:
| Wheel | Backspacing | Outer lip from hub face |
|---|---|---|
| 8.5 in wide, offset +35 mm | 6.13 in | 3.37 in |
| 9 in wide, offset −12 mm | 4.53 in | 5.47 in |
| 10 in wide, offset 0 mm | 5.50 in | 5.50 in |
| 8 in wide, offset −19 mm | 3.75 in | 5.25 in |
| 7.5 in wide, offset +45 mm | 6.02 in | 2.48 in |
What happens when you change them
- Same width, 10 mm lower offset. The whole wheel moves 10 mm outward. Both lips shift by the same amount.
- One inch wider, same offset. The wheel grows half an inch each way. The inner lip moves 12.7 mm toward the suspension and the outer lip moves 12.7 mm outward.
- One inch wider, keeping the outer edge where it is. You raise the offset by 12.7 mm. All of the extra width then goes inward, which is 25.4 mm closer to the suspension. That is why a wider wheel is usually limited by the room on the inside.
Offset also affects steering geometry and the load on wheel bearings. A change of a few millimetres is rarely a concern, but large changes alter steering feel and stress parts designed around the original offset.
Measuring your own
- Look for ET and a number on the back of the wheel, along with the width (such as 8.5J). Not every wheel is marked, and aftermarket wheels may show backspacing instead.
- To measure backspacing, lay the wheel face down. Rest a straightedge across the back of the wheel and measure from the straightedge down to the mounting surface.
- Check that the bolt pattern and center bore match your hub too. Offset is only one of the numbers that has to be right.
Enter a current wheel and a new one to see how far each lip moves, and enter your own measurements to compare against the room on your vehicle.Open the calculator